Ultrasound imaging system and method for improving resolution and operation

ABSTRACT

An ultrasound system and method are provided for improving resolution and operation. The system applies different imaging parameters within and outside a region of interest in an ultrasound image to improve spatial and/or temporal resolution inside a region of interest. The system also increases an apparent frame rate within a region of interest in an ultrasound-image frame by generating a motion-compensated interpolated image based on measured motion. The ultrasound imaging system also performs a method for automatically adjusting ultrasound imaging parameters in at least a portion of an ultrasound image in response to transducer or image motion to improve spatial or temporal resolution. With the measured motion, the system can also alter an operating mode of an ultrasound transducer array in response to an absence of transducer motion. Further, the system corrects distortion in an acquired ultrasound image caused by transducer or image motion.

BACKGROUND OF THE INVENTION

There are several limitations of conventional ultrasound imaging systemsthat result in reduced spatial and temporal resolution in generatedultrasound images. One limitation is that conventional systems do notoffer much flexibility in controlling imaging parameters on anintra-frame or regional basis. For example, many imaging parameters suchas filtering, operating frequency, gain, number of transmit foci, andline density are uniform across the image frame, although line densitymay follow a predetermined reduction toward the sides of the image.Other imaging parameters may be controlled on a regional basis in a verylimited manner. For example, gain in the axial or the lateral imagingdimension can be controlled in a region by using slide potentiometersfor each depth in the image. The Regional Expansion® feature and thezoom feature found in commercial imaging systems also provide a verylimited amount of regional control. The Regional Expansion® featureincreases line density within a region to enhance spatial resolution,but the area outside the region is not scanned. The zoom feature merelymagnifies a region, typically to fill the display screen. Additionally,modes such as color flow velocity, energy, or combined energy andvelocity modes with a B-mode display can operate in a particular regionof the image.

Another limitation of conventional ultrasound imaging systems is thatthey typically suffer from low frame rates. Because ultrasound systemstypically present one image for every set of lines acquired, there is atradeoff between spatial resolution and frame rate. That is, if a systemis performing deep, detailed imaging requiring, for example, high linedensity and multiple transmit foci, a greater amount of time is requiredto acquire a set of lines. This increased amount of time can result in aframe rate that will be uselessly low. For example, with 256 lines, animaging depth of 300 mm, and 5 transmit focal zones, the frame rate ismerely two frames per second. In many areas (such as cardiology), aseverely compromised frame rate is unacceptable. While repeatedlydisplaying the generated frames can match the video refresh rate (30frames per second in the United States), repeated frames provide no newinformation to the user.

A third limitation of conventional ultrasound imaging systems is due tothe complexity of spatial and temporal controls. Typically, a userquickly scans a patient to find a particular area of interest and thenslowly scans that area to acquire a detailed image. Although the optimalimaging parameters are different in a fast scan than in a slow scan,many users choose not to fully optimize these parameters becauseadjusting imaging controls is often cumbersome.

A fourth limitation concerns transducer overheating. With some imagingmodes, such as Color Doppler, the transducer will reach an elevatedtemperature if, while the ultrasound system is powered, the transduceris not used for imaging. Elevated temperatures can be damaging to thetransducer and have been implicated as a contributing factor to reducedprobe life. Elevated temperatures are also undesirable to a patient if ahot probe is applied to the patient. Some approaches to solving thisproblem include attaching a magnetic position sensor or an accelerometerto the transducer probe to automatically turn off the probe when nomotion is sensed--an indication that the probe is not in use. Thesesensors are typically expensive (partly because they offer more featuresthan are needed to solve the problem) and require modifications to bemade to the probe. Other methods involve using latched probe holders,but to be effective, these methods require the user to place the probein the proper holder. There is, therefore, a need for an inexpensivealternative that does not require user intervention.

A fifth limitation of conventional ultrasound imaging systems is thatthe displayed image typically exhibits geometric distortions due toimage or transducer motion during the acquisition of an image frame.Scan lines in an ultrasound frame are acquired sequentially--notsimultaneously. Accordingly, a finite amount of time elapses between theacquisition of the left-most line and the right-most line in an imageframe. Image or transducer motion after the ultrasound system acquiresthe left-most line but before it acquires the right-most line can resultin a distorted image. Distortions can also be caused by high transmitfocus zone formats and in the mixed B-mode/Color Flow modes where thereis a significant time delay between B-Mode lines acquired on the leftand right hand side of the color box. An additional distortion occurswhen transducer motion results in a scan line being fired at a physicallocation that corresponds to a location outside the image frame.

There is, therefore, a need for an ultrasound system and method thatwill overcome the problems described above.

SUMMARY OF THE INVENTION

The present invention is directed to an ultrasound system and method forimproving resolution and operation. According to a first aspect of thisinvention, an ultrasound imaging system performs a method for improvingspatial characteristics within a region of interest within an ultrasoundimage. The method comprises the steps of selecting a region of interestin an ultrasound image; selectively applying a first set of imagingparameters inside the region of interest to improve spatial resolutioninside the region of interest, said first set being different from asecond set of imaging parameters applied outside the region of interest;and assembling a composite image comprising a first image portion withinthe region of interest and a second image portion outside the region ofinterest, said first and second image portions being in the same imagingmode.

Imaging parameters that can be selectively applied comprise one or moreof the following: (a) line density, (b) transmit foci per scan line, (c)pre-detection filter characteristics, (d) post-detection filtercharacteristics, (e) post-processing map characteristics, (f) ultrasoundoperating frequency, (g) transmit power, (h) logarithmic compressionprofiles, (i) multiple receive lines per transmit line, (j) transmitpulse shape, and (k) receive frequency band.

According to a second aspect of this invention, an ultrasound imagingsystem performs a method for improving temporal characteristics within aregion of interest within an ultrasound image. The method comprises thesteps of selecting a region of interest of an ultrasound-image frame;selectively applying a first set of imaging parameters inside the regionof interest to improve temporal resolution inside the region ofinterest, said first set being different from a second set of imagingparameters applied outside the region of interest; and assembling acomposite image comprising a first image portion within the region ofinterest and a second image portion outside the region of interest, saidfirst and second image portions being in the same imaging mode.

Temporal resolution can be improved, for example, by increasing anactual frame rate inside the region of interest by acquiring additionalreal ultrasound-image frames inside the region of interest.Motion-compensated interframe interpolation can match the frame rate ofthe areas inside and outside the region of interest by measuring motionbetween a first and a second ultrasound-image frame outside of theregion of interest and generating a motion-compensated interpolatedframe. Alternatively, a real ultrasound-image frame can be generatedoutside the region of interest if an accurate measure of motion cannotbe made.

Another way in which temporal resolution can be improved is by using adifferent persistence level inside the region of interest than outsidethe region of interest. Persistence can be varied as a function ofmeasured motion within the region of interest.

Yet another way in which temporal characteristics can be varied is byincreasing an apparent frame rate of an ultrasound imaging system. Toaccomplish this, the imaging system can perform a method comprising thesteps of manually or automatically selecting a region of interest of anultrasound image, measuring motion between a first and secondultrasound-image frame in a region of interest, generating amotion-compensated interpolated image inside the region of interestbased on the measured motion, and then inserting the motion-compensatedinterpolated image between the first and second ultrasound-image framesinside the region of interest.

According to a third aspect of this invention, a method forautomatically adjusting ultrasound imaging parameters in anultrasound-image frame in response to transducer or image motioncomprises the steps of measuring motion and then automatically applyingimaging parameters in a region of interest in an ultrasound image inresponse to measured motion. Motion can be measured by a motion sensorin a transducer or by measuring motion of a sub-block of pixels betweenat least two ultrasound-image frames. Imaging parameters in the regionof interest can be automatically applied to improve spatial or temporalresolution in response to measured motion being below or above athreshold value, respectively.

According to a fourth aspect of this invention, a method is provided forautomatically altering an operating mode of an ultrasound transducerarray in response to an absence of transducer motion to reduce the riskof transducer heating. This method comprises the steps of measuringmotion of a transducer array by measuring motion of a sub-block ofpixels between at least two ultrasound-image frames and automaticallyaltering an operating mode of a transducer array in response to anabsence of measured motion. To alter the operating mode in response toan absence of measured motion, an ultrasound system can remove or reducepower applied to the transducer array or disable imaging modes thatcause elevated temperatures in the transducer array.

According to a fifth aspect of this invention, a method for correctingdistortion caused by transducer or image motion in a region of interestin an ultrasound-image frame comprises the steps of measuring motion andautomatically generating a distortion-corrected image inside a region ofinterest in response to the measured motion. Motion can be measured by amotion sensor in a transducer or by measuring motion of a sub-block ofpixels between at least two ultrasound-image frames. Adistortion-corrected image can be generated by estimating an effect ofthe measured motion on line spacing and reprocessing line data withcorrected line spacing. A distortion-corrected image can also begenerated by repositioning sub-blocks of pixels in response to motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ultrasound imaging system of a preferredembodiment.

FIG. 2 is a flow chart of a method for forming an enhanced image withinan ultrasound-image frame.

FIG. 3 is a flow chart illustrating how the controller of a system ofthe preferred embodiment of FIG. 1 controls the system to apply imagingparameters and assemble a composite image.

FIG. 4A shows an operational block diagram for creating an output imageframe using a persistence filter coefficient in a selected region ofinterest.

FIG. 4B is a graph which illustrates characteristics of three differentfilter designs which can be used to generate a persistence filtercoefficient.

FIG. 5 illustrates a technique for spatially smoothing a compositeimage.

FIG. 6 is a block diagram of a first alternative ultrasound imagingsystem.

FIG. 7 is a flow chart illustrating how a controller of the firstalternative system controls the system to vary imaging parameters.

FIG. 8 is a block diagram of a second alternative ultrasound imagingsystem.

FIG. 9 is a block diagram of an ultrasound imaging system of anotherpreferred embodiment.

FIG. 10 is a flow chart illustrating a method for increasing an apparentframe rate within a region of interest in an ultrasound image bygenerating motion-compensated interpolated images.

FIG. 11 is a block diagram of an ultrasound imaging system of anotherembodiment.

FIG. 12 is a flow chart illustrating a method for automaticallyadjusting imaging parameters in a region of interest in an ultrasoundimage in response to detected transducer or image motion.

FIG. 13 is a block diagram of an ultrasound imaging system of anotherpreferred embodiment.

FIG. 14 is a flow chart of a method for generating adistortion-corrected image inside a region of interest in response tomeasured image or transducer motion.

FIG. 15 is an illustration of an ultrasound image frame in whichsub-block P is located at position X1.

FIG. 16 is an illustration of an ultrasound image frame in whichsub-block P is located at position X2.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS COMPOSITEIMAGE EMBODIMENTS

Turning now to the figures, FIG. 1 shows a block diagram of anultrasound imaging system 110. The ultrasonic imaging system 110includes a controller 115 with a user interface 120. The controller 115is coupled to a transmit beamformer 125, which is coupled to atransducer array 135. As used herein, "coupled to" means directlycoupled to or indirectly coupled through one or more components.Similarly, "responsive to" means directly responsive to or indirectlyresponsive through one or more components.

This system 110 further comprises a first and second signal path 140,145, comprising a first and second processor 150, 155, a first andsecond line memory 160, 165, and a receive beamformer 130. While FIG. 1shows the first and second signal paths 140, 145 sharing one beamformer130, it is important to note that each signal path can contain its ownreceive beamformer. The first and second signal paths 140, 145 areresponsive to the transducer array 135 and are coupled to imagecombination logic 170. The image combination logic 170 is coupled to ascan converter 175, which is coupled to an image memory 180 and adisplay 185. Motion estimation logic 190 is responsive to the imagememory 180 and is coupled to the controller 115, which is coupled toeach component of the system 110.

As with conventional ultrasound systems, this system 110 performsultrasonic visualization, the well-known interrogating-and-imagingprocess which includes ultrasound generation, ultrasound detection,image reconstruction, and image presentation phases. During theultrasound generation phase, the transmit beamformer 125 appliesmultiple signals to elements of a transducer array 135 to cause theelements to vibrate and emit ultrasonic energy to a tissue. Next, in theultrasound detection phase, the receive beamformer 130 measures thesignals created by the transducer array 135 when ultrasonic energyreflected by the structures in the tissue impinge on the transducerarray 135.

The signals generated by the receive beamformer 130 are channeled to thescan converter 175 for image reconstruction. During this phase, the scanconverter 175 processes the detected signals to create an image, whichis presented on the display 185 during the image presentation phase.Additionally, the image can be stored in the image memory 180.

The controller 115 controls the operation of the components of thesystem 110. A user, via the user interface 120, can adjust imagingparameters such as, but not limited to, image depth, image width, andframe rate. The controller 115 interprets the set-up information enteredby the user and configures the components of the system 110 accordingly.Alternatively, the controller 115 can establish imaging parameterswithout input from the user.

Unlike conventional ultrasound systems, this system 110 comprises twosignal paths 140, 145 and motion estimation logic 190. With thesecomponents and appropriate programming in the controller 115, thissystem 110 comprises means for selecting a region of interest within anultrasound-image frame; means for selectively applying a first set ofimaging parameters inside the region of interest, said first set beingdifferent from a second set of imaging parameters applied outside theregion of interest; and image assembly means for assembling a compositeimage comprising a first image portion within the region of interest anda second image portion outside the region of interest, said first andsecond image portions being in the same imaging mode. As used herein,"set" includes one and more than one member.

FIG. 2 is a flow chart illustrating a method for improving spatialcharacteristics within a region of interest in an ultrasound image.First, a region of interest in an ultrasound image is selected (step210). Next, the system selectively applies a first set of imagingparameters inside the region of interest to improve spatial and/ortemporal resolution inside the region of interest, said first set beingdifferent from a second set of imaging parameters applied outside theregion of interest (step 220). Then, a composite image comprising afirst image portion within the region of interest and a second imageportion outside the region of interest is assembled (step 230). Bothimage portions can be in the same imaging mode (e.g., both imageportions can be B-mode images, both can be color Doppler images, and soforth). The steps of this method, which will be described in more detailbelow, can be performed in real time.

Selecting a Region of Interest

The first step (step 210) of this method is to select a region ofinterest in an ultrasound image. The region of interest can comprise theentire ultrasound image or a portion thereof and covers a selected setof scan lines and at least a portion of range direction for the selectedset of scan lines. The region of interest can be rectangular for alinear format and arc-shaped for a sector/Vector® format.

A user can manually select the region of interest though the userinterface 120. For example, a user can use a track ball to positiongraphical lines on the display 185 to mark the region of interest arounda detailed and fast moving object, such as a heart valve leaflet. Theuser can also use an "Image Width" function found in some commerciallyavailable ultrasound imaging systems to designate the region ofinterest.

Alternatively, the region of interest can be automatically selected(e.g., by the controller 115) to surround regions of motion. Forexample, motion estimation logic 190 performing an image motion trackingtechnique can determine frame-to-frame motion of a sub-block of pixels.If the detected motion of the sub-block exceeds a preset or adjustablethreshold level, the controller 115 can select the region of interest tosurround that sub-block. The components and function of the motionestimation logic 190 are described in detail below in connection withthe motion compensation embodiments.

More than one independent region of interest can be selected. Forexample, regions of interest can be selected, either manually orautomatically, around fast moving, slow moving, and intermediate movingportions of the image. Due to the computational complexity involved inusing multiple regions of interest, it is presently preferred that thenumber of selected regions of interest be limited to one or two.

Selectively Applying Imaging Parameters

Once the region of interest is selected, a first set of imagingparameters can be selectively applied inside the region of interest anda different set of imaging parameters can be applied outside the regionof interest either manually by the user via the user interface 120 orautomatically by the controller 115 (step 220). FIG. 3 is a flow chartshowing how the controller 115 controls the operation of the system 110to apply imaging parameters inside and outside the region of interest.First, a given line and range location is analyzed by the controller 115to determine whether the location is within the selected region ofinterest (step 310). If the location is within the selected region ofinterest, the controller 115 signals the appropriate component (e.g.,the beamformers 125, 130, the second processor 155 in the second signalpath 145, etc.) to apply a first set of imaging parameters (step 320).For example, for increased line density inside the region, thecontroller 115 would instruct the transmit beamformer 125 to fire morelines and instruct the receive beamformer 130 to receive more lineswithin the region of interest.

If the location of the line and range is outside of the selected regionof interest, the controller 115 allows the ultrasound data correspondingto that location to pass through the first signal path 140 and a second,different set of imaging parameters is applied (step 330). It ispreferred that the image within the region of interest and the imageoutside the region of interest be in the same imaging mode (e.g.,B-mode, color mode, etc.). The signals from the first 150 and second 155processors are held in the first 160 and second 165 line memories,respectively (steps 340 and 350). The other steps in the flow chart arediscussed below. As mentioned below, the above-described steps arerepeated for every line and range location in the image (step 380).

As mentioned above, the controller 115 can apply imaging parameterswithin the region of interest to improve both spatial and temporalresolution within the region of interest. Several imaging parameters canbe used, either individually or in combination with one another, toimprove spatial resolution. For example, different line densities can beused inside and outside the region of interest. In general, a greaterline density provides better spatial resolution (which is desirable whenfine structures need to be resolved) but typically results in lowertemporal resolution (frame rate). With the system 110 of this preferredembodiment, the controller 115 can instruct the transmit 125 and receive130 beamformers to send and receive more ultrasound beams within theregion of interest than outside the region of interest.

Another imaging parameter that can be applied is the number of transmitfoci per scan line. In general, an ultrasound beam has the smallestazimuthal beam width at the transmit focus. Away from the focus, thebeam width widens. Since smaller azimuthal widths result in betterazimuthal resolution, it is preferred to have small width throughout therange of the region of interest. Increasing the number of transmit fociper ultrasound line will increase the number of locations where the beamwidths are small. The impact of having more transmit foci per line isthat these lines are transmitted and received from the same directionfor as many times as there are number of foci. Increasing the number oftransmit foci per line is well known in the art. Generally, thecontroller 115 instructs the beamformers 125, 130 either to transmit andreceive multiple lines from the same spatial location or to advance inthe scanning operation.

Ultrasound operating frequency is another imaging parameter that can beapplied inside the region of interest. The choice of transmitfrequencies involves a tradeoff between spatial resolution andpenetration--a higher frequency improves spatial resolution and a lowerfrequency improves penetration. This tradeoff can be particularly usefulwhen, for example, better spatial resolution is desired inside theregion of interest and higher penetration is desired outside the regionof interest. The controller 115 controls the frequency being transmittedand the timing of the transmission and additionally synchronizes theother components of the system 110 accordingly.

A different transmit power can also be used within the region ofinterest. This is especially important when using harmonic-generatingagents or conventional contrast agents. If the transmit power is toohigh, it will destroy the contrast bubbles. If the transmit power is toolow, penetration and signal-to-noise ratio will decrease and the secondharmonic (non-linear) operation will not activate. By selecting theregion of interest around the area that contains the agents, the usercan ensure optimal operation. To implement this variation, thecontroller 115 instructs the transmit beamformer 125 to use differentpower within the region of interest. The controller 115 preferablyadjusts the receive gain in the receive beamformer 130 according to thetransmit power to achieve a uniform brightness throughout the scanplane.

Additionally, a logarithmic compression profile in a particular regionof interest can be altered. The system 110 has a limited dynamic range,defined as the range between the largest magnitude of signal the system110 can process and the noise floor set by the system's 110 electronics.Variations in log compression profile determine whether the entire rangeor a portion of the range is displayed. For example, if a user is onlyinterested in bright targets in a region of interest, a small logcompression profile displays only the large magnitude signals in theregion of interest. If a user is interested in seeing both bright andweakly reflecting targets in the region of interest, a large logcompression profile displays both the large and small magnitude signals.

Another imaging parameter that can be varied is post-processing maps.Post-processing maps transform the scan-converted data to output valuesthat are finally mapped onto the display. Changing post-processing mapsalters the range of input signal that are emphasized. Additionally, thenumber of receive lines per transmit line can vary. Multiple lines ofinformation can be received from slightly different directions followinga single transmit operation. Receiving multiple lines of informationprovides better spatial resolution. In operation, the controller 115instructs the receive beamformer 130 to receive multiple lines.

Pre- and post-detection filter characteristics can also be varied.Received RF signals can be summed in various ways and can be transformedto IF or baseband signals before summation. The envelope of the summedsignal is extracted and stored for scan conversion. Filters can beapplied to the summed IF or baseband signals to enhance certain aspectsof the image prior to detection. For example, a nonlinear filer based onsignal intensity can be applied to smooth-out low-level noise. Differentfiltering characteristics can be applied prior to or after scanconversion and can be controlled as a function of depth.

Different transmit pulse shapes can also be used. A transmit pulse shaperefers to the number of cycles in the transmit burst. It also refers toa different Gaussian pulse bandwidth. In general, a transmit pulse shapecan be chosen for the region of interest to reduce signal attenuation asthe signal propagates through tissue.

Additionally, a different receive frequency band can be used within theregion. For example, harmonic frequencies (frequencies associated withnon-linear propagation or scattering of transmit signals) can bereceived within the region of interest. As used herein, harmonicincludes subharmonics as well as second, third, fourth, and otherharmonics. Second harmonic frequencies produce ultrasound images withdramatically reduced acoustic clutter and better beamforming than thefundamental image counterpart.

In addition to or in combination with applying imaging parameters toimprove spatial resolution, imaging parameters can be applied to improvetemporal resolution within the region of interest. One way in whichtemporal resolution can be improved is by increasing the frame rateinside the region of interest by acquiring additional real (i.e.,non-interpolated) ultrasound-image frames within the region of interest.It is preferred that real frames be acquired for regions that containcontrast agents, especially those that radiate energy at the secondharmonic.

Temporal resolution can also be improved by using a differentpersistence level inside the region of interest than outside the regionof interest. Persistence inside the region of interest can be a functionof image motion. As used herein, the term "image motion" refers tomotion within an ultrasound image such as, but not limited to, tissuemotion and motion of contrast agents. As described in more detail below,motion can be detected by using motion estimates of a sub-block ofmoving pixels or by computing the difference between pixels at the samespatial location in successive frames. If there is significant imagemotion, it is preferred that persistence be reduced to avoid smearing orblurring of the moving object. Similarly, if there is very little imagemotion, it is preferred that persistence be increased to average outnoise in the image, thereby increasing signal-to-noise ratio. Asdiscussed below, persistence inside the region of interest can be variedusing the techniques of motion-compensated persistence withmotion-compensated interpolated frames, motion-compensated persistencewith real ultrasound frames, and motion-adaptive persistence.

One way in which persistence inside the region of interest can be variedis by using motion-compensated persistence with motion-compensatedinterpolated frames. (The generation of motion-compensated interpolatedimage regions is described in the Motion Compensation Embodimentssection below.) In this technique, interpolated frames are generated tospatially coordinate the position of moving objects in the image. Forexample, suppose an output image frame comprises a summation of threeimage frames--frame N, frame N-1, frame N-2. Also suppose that themotion estimation logic 190 detects object motion of 4 pixels to theright in each of the image frames. Motion-compensated interpolated imageframes can be generated, as described below, so that the three framesbeing weighted are frame N, frame N-1 with the moving object translated4 pixels to the right, and frame N-2 with the moving object translated 8pixels to the right. In this way, the moving object in each of the threeframes would be located in the same spatial position before summing thethree frames to generate an output frame, thereby avoiding the problemof object blurring. While only one moving object was used in the exampleabove, it is important to note that different motion vectors can becomputed for different parts of the image so several moving objects canbe similarly analyzed. In this way, in addition to avoiding the problemof object blurring, noise will be reduced in the same amount in bothmoving and non-moving areas of the image.

Another way in which persistence inside the region of interest can bevaried is by using motion-compensated persistence with real ultrasoundframes. This technique is similar to the first technique described abovein that motion vectors can be computed for different parts of the image,but instead of generating a motion-compensated interpolated frame, thepersistence filter determines which pixels to process. That is, in thefirst technique, motion estimates are used to create motion-compensatedinterpolated frames to align moving objects in an image. The persistencefilter in the first technique processes pixels corresponding to the samespatial location across a number of frames. In this technique, thespatial locations employed by the persistence filter are determined bythe motion estimates. That is, motion estimates are used to determinethe location of a moving block of pixels, and that location is used bythe persistence filter. In this way, pixels that belong to the sameobject are filtered. When there is no motion is present, the filter usesthe same spatial location, as in the case of a conventional persistencefilter. As with the first technique, this technique provides theadvantages of reducing blur and of reducing noise in the same amount inboth moving and non-moving areas of the image. Because motion estimatesemployed in this technique, the same motion estimates can be used togenerate motion-compensated interpolated frames, thereby reducingcomputation time by avoiding recomputation of motion estimates.

It is important to note that in both the first and second technique, anon-recursive procedure (i.e., each output frame being a weighedcombination of input and previous output frames) can be used. It is alsoimportant to note that in both the first and second technique, thepersistence filter coefficient in all or part of the image frame canvary as a function of image motion.

The third technique that can be used to vary persistence as a functionof image motion is motion-adaptive persistence. Motion-adaptivepersistence can be used with the techniques described above to vary thepersistence filter coefficient in all or part of the image frame as afunction of image motion. Motion-adaptive persistence can also be usedwith other persistence techniques. Motion-adaptive persistence will bedescribed in reference to FIG. 4A, which shows an operational blockdiagram of a persistence filter using a recursive procedure. As shown inthis figure, an output frame O(n) is generated by adding the intensityof each pixel in a region in the previous output frame O(n-1),multiplied by a persistence filter coefficient α(n), with the intensityof each corresponding pixel in a region of the input frame I(n),multiplied by (1-α(n)). The persistence filter coefficient α(n) can berecalculated on a frame-by-frame basis or repeated after a number offrames (e.g., once every 10 frames).

A persistence-filter-coefficient generator 450 generates a persistencefilter coefficient α(n) for the region of interest from the previousultrasound-image frame O(n-1) and the current ultrasound-image inputframe I(n). Next, a first multiplier 470 multiplies the persistencefilter coefficient α(n) with previous ultrasound image frame O(n-1) toform a modified previous ultrasound image frame O(n-1) in a region ofinterest. A second multiplier 475 multiples the input ultrasound imageframe I(n) with (1-α(n)) to create a modified input frame I'(n).Finally, adder 490 adds the pixel intensity of each pixel in the regionof interest in the modified input frame I'(n) with the pixel intensityof each pixel in the region of interest in the previous ultrasound-imageframe O(n-1) to generate an enhanced region of interest in the currentultrasound-image output frame O(n). The operations shown in this diagramcan be implemented in the controller 115, for example.

In using motion-adaptive persistence, the persistence filter coefficientα(n) varies as a function of image motion. Specifically, the persistencefilter coefficient α(n) increases as the level of motion within theregion decreases. FIG. 4B illustrates such a relation for threedifferent filter designs 492, 494, 496, although others designs can beused. As shown in FIG. 4B, the persistence filter coefficient α(n)increases as d decreases. The function d can be computed by using motionestimates of a sub-block of moving pixels or by computing the differencebetween pixels at the same spatial location in successive frames. Thefunction d preferably is derived from motion estimates of sub-blocks ofpixels determined to give the best match between a region in theprevious frame O(n-1) and the current image frame I(n). This motionvector is the value (x,y) which gives the minimum sum of absolutedifferences and may be derived at high speed using a L64720A motionestimator or a similarly programmed TMS320C80 processor. Technically,the net motion length is the square root of (x² +y²), where x and y arethe pixel shifts required to obtain the best match.

An advantage of this implementation is that the sum of absolutedifferences is an error signal related to noise in the image. If thedetected motion is small or varies randomly between sequences and thesum of absolute differences is larger than a threshold, the image isprobably stationary and noisy. Persistence could then, accordingly, beincreased.

As mentioned above, the function d can also be computed by computing thedifference between pixels at the same spatial location in successiveframes--an indication of image movement. That is, if motion is present,it is likely that the pixel values will change from frame to frame.Specifically, the function d can be given by the following equation:##EQU1## wherein I(n,x,y) comprises an intensity value of a pixel in thecurrent ultrasound-image input frame, O(n-1,x,y) comprises an intensityvalue of a pixel in a previous ultrasound-image output frame, and "(x,y)in A(i,j)" comprises every pixel (x,y) in an area A(i,j) in the selectedregion of interest. It is important to note that other equations can beused to represent the function d.

There are several alternatives. First, instead of comprising every pixelin the region of interest, area A(i,j) can comprise a subset of thepixels in the region of interest to reduce the amount of computation. Inthis alternative, area A(i,j) can be located around the center of theregion or can comprise pixels that have an intensity amplitude greaterthan a predetermined threshold. Even though some of the pixels were notused in computing the coefficient α(n), the generated persistence filtercoefficient α(n) can still be applied to all pixels in the region.

As another alternative, more than one region can be selected. Forinstance, the user can select more than one region of interest.Preferably, if multiple regions are required, selection is based onautomatic measurement, for example, of regions with low or high motion.Alternatively, the entire image frame can be divided into regions, andthe calculation and applications discussed above can be applied over theentire image frame through each of the regions. If the persistencefilter coefficient α(n) is computed for multiple regions, the particularpersistence filter coefficient applied to a specific pixel in the framecan be computed by linearly interpolating among the persistence filtercoefficients of adjacent regions based on, for example, the pixel'sdistance from a particular point (e.g., the center) of each region.

There can be situations in which the value of d is high even though mostpixels in the region are below a certain intensity value. Such asituation arises due to noise in the region, not object motion. If theminimum-sum-of-absolute-differences method is used, situations in whichd is small or randomly varying and the sum-of-absolute differences islarge indicate noise and little motion. In these situations, thepersistence filter coefficient α(n) can be assigned a high value toaverage out the noise, thus improving the signal-to-noise ratio for theregion.

There are several advantages to varying persistence as a function ofimage motion. First, by reducing persistence when the image exhibitsmotion, spatial resolution is preserved. Similarly, by increasingpersistence when the image exhibits little or no motion, thesignal-to-noise ratio for the region is increased by averaging outnoise. By using a plurality of regions across the image frame, thesignal-to-noise ratio can be improved over stationary and high noiseregions, while preserving the spatial resolution in regions of the frameexhibiting motion. Additionally, the tissue motion imaging modes such asDTV and DTE are improved by preventing blackouts when a tissue isstationary.

Assembling the Composite Image

After the different sets of imaging parameters are applied inside andoutside the region of interest, the image combination logic 170 combinesa first image portion within the region of interest with a second imageportion outside the region of interest to form a composite image, asshown in step 230 in FIG. 2 and in step 360 in the flow chart of FIG. 3.If a particular line and range location is within the selected region ofinterest, the image combination logic 170 selects data from the secondsignal path 145. Alternatively, if a particular line and range locationis outside the selected region of interest, the image combination logic170 selects data from the first signal path 140. In this way, the imagecombination logic 170 controls what image data will be sent to the scanconverter 175.

Lastly, the scan converter 175 converts the ultrasound data onto an x-ydisplay grid and displays the converted information on the display 185and/or stores the converted information in the image memory 180 (step370). This process is repeated for every line and range location in theimage (step 380). At the completion of this process, the display 185presents a composite image comprising a first image portion within theregion of interest and a second image portion outside the region ofinterest. The image combination logic 170 and the controller 115 canadditionally comprise hardware or software to present a temporally andspatially smooth composite image, as described below.

Temporal Smoothing

When the actual frame rate inside the region of interest is greater thanthe actual frame rate outside the region of interest, the apparent framerate of the image outside the region of interest can be increased togive the composite image the appearance of having a single frame rate.To match the frame rates, the ultrasound system 110 inserts interpolatedframes between real frames outside the region of interest.

First, the motion estimation logic 190 measures motion between at leasttwo real ultrasound-image frames outside of the region of interest bydetecting frame-to-frame motion of a sub-block of pixels. Next, themotion estimation logic 190 generates motion-compensated interpolatedframes outside the region of interest, which are then inserted betweenthe real ultrasound image frames. This method is more completelydescribed below in the Motion Compensation Embodiments section.

By using this method to give the composite image the appearance ofhaving a single frame rate, considerable time savings are obtained whencompared to the time needed to acquire a real image for the entireframe. For example, if a selected region of interest comprises 10% ofthe total image dimension and the time required for acquiring a realframe is 100 ms, 10 ms are needed to capture an additional real imagewithin the region of interest. Hence, the time needed to capture tworeal frames for the entire image dimension (100 ms+100 ms 200 ms, or 100ms per final displayed frame) is considerably greater than the timeneeded to capture one real frame for the entire image dimension and anadditional real frame within the region of interest (100 ms+10 ms=110ms, or 55 ms per final displayed frame), assuming that the time requiredfor frame interpolation is negligible.

This method of temporally smoothing the composite image can also beapplied with more than one independent region of interest. For example,if three regions are selected and the frame intervals of the regions areenhanced to 200 ms, 100 ms, and 50 ms, respectively, then the compositeimage should have an apparent overall frame rate matching the highestactual frame rate (50 ms for this example). The apparent frame rate ofthe non-enhanced image outside of the regions is increased as describedabove. Additionally, interpolated frames can be inserted into the twoother regions of interest to increase their apparent frame rate to 50ms.

In performing the temporal smoothing method described above, if anaccurate measurement of motion cannot be made, the ultrasound system canacquire additional real ultrasound-image frames instead of generating aninterpolated frame. The accuracy of the measurement can be assessed bycomparing a pixel error signal of the measured motion against athreshold level, as will be described in greater detail below.

Instead of using motion-compensated interpolated frames to affecttemporal smoothing, the image data outside the region of interest can berepeated to match the frame rate of the image inside the region ofinterest. Unlike the method of temporal smoothing usingmotion-compensated interpolated frames, this method does not present newimage information to the user. Technically, it is necessary to delay thedisplay of frames so that the interpolated frames can be calculated fromthe frame before and the frame after. However, the delay of a couple offrames will not generally be noticeable to the user.

Spatial Smoothing

To form a spatially smooth composite image, a smooth transition can becreated between the first and second image portions. A smooth transitioncan be formed by summing a fraction of the first image portion with afraction of the second image portion at the boundaries of the region ofinterest. Referring now to FIG. 5, preferably, in the region 530, thepixel locations closest to region 510 are determined by a weighted sumof image data from the first and the second image data, wherein thefirst image data is emphasized, while at a location closest to region520, the second image data is emphasized (i.e., linear interpolation ofthe first and second data depending on position). Another way tospatially smooth the composite image is to apply a low-pass filter (forexample, within four pixels from the boundary) to reduce artifacts.Additional artifact reduction methods are described below.

There are several alternatives to the above-described system 110. FIG. 6shows one alternative system 610. The two systems 110, 610 have severalcommon components, which will not be further described here. However, inaddition to the receive beamformer 630, the signal paths 640, 645 ofthis system 610 contain a first and second processor 650, 655 and afirst and second scan converter 660, 665 respectively. FIG. 7 is a flowchart showing how the controller 615 controls operation of the system610 to selectively apply sets of imaging parameters.

First, ultrasound image data is processed through the first 640 andsecond 645 signal paths (step 710). In the first signal path 640, afirst set of image data for the entire scan region is processed by thefirst processor 650 and scan converted onto an x-y display grid by thefirst scan converter 660. In the second signal path 645, a second set ofimage data within the region of interest is processed by the secondprocessor 655 and scan converted onto an x-y display grid by the secondscan converter 665. As with the above-described system 110, thecontroller 615 analyzes a given line and range location to determinewhether it is within the selected region of interest and, accordingly,what set of imaging parameters to apply. Next, the image combinationlogic 670 stores the scan-converted, first set of image data in theimage memory 680 (step 720). The image combination logic 670 thenselects the memory address of the region of interest and writes thescan-converted image corresponding to the second set of image data fromthe second signal path 645 over the stored image from the first signalpath 640 in that region (step 730) to form a composite image. Thiscomposite image can then be presented to the user via the display 685and/or stored.

FIG. 8 shows a second alternative system 810 to the above-describedultrasound imaging system 110. The two systems 110, 810 also haveseveral common components, which will not be further described here. Inthis system 810, however, only one signal path, which contains aprocessor 890, is used. As with the above systems 110, 610, thecontroller 815 analyzes a given line and range location to determinewhether it is within the selected region of interest. Based on thatdetermination, the controller 815 signals the processor 890 to applycertain imaging parameters.

It is important to note that in all of the above-described systems, aswell as the systems of the preferred embodiments below, additionalcomponents may be added to the system and described functions can beimplemented with one or more components of the system.

MOTION COMPENSATION EMBODIMENTS

FIG. 9 is a block diagram of an ultrasound imaging system 910 thatcomprises means for generating a motion-compensated interpolated imageinside a region of interest in response to measured image motion. Asmentioned above, the term "image motion" refers to motion within anultrasound image such as, but not limited to, tissue motion and motionof contrast agents. This system 910 is configured similarly to that ofthe alternate system 810 of the composite image embodiments. FIG. 9additionally shows the components of the motion estimation logicdescribed in the composite image embodiments. Specifically, an imagesub-block selector 940 is provided, which is responsive to thecontroller 915 and is coupled to a first 945 and second 950 imagebuffer. The first 945 and second 950 image buffers are responsive to theimage memory 980 and are coupled to a motion estimator 955. The motionestimator 955 is coupled to a motion smoother 960, which is coupled to amotion scaler 965. An image interpolator 970 is responsive to the motionscaler 965 and the first image buffer 945 and is coupled to the display985. The specific function of these components will be described below.

The system 910 of this preferred embodiment can implement a method forincreasing the apparent frame rate within a region of interest in anultrasound image by generating motion-compensated interpolated images,as illustrated in FIG. 10. First, a region of interest in anultrasound-image frame is selected (step 1010). Next, motion within theregion of interest is measured (step 1020). Based on this measuredmotion, a motion-compensated interpolated image is generated andinserted between real ultrasound-image frames inside the region ofinterest (step 1030). By using this method, the ultrasound system 910increases the apparent frame rate within the region of interest. Thesteps of this method, which will be described in more detail below, canbe performed in real time.

First, a region of interest in an ultrasound-image frame is selected(step 1010). The region of interest can comprise all or a portion of theultrasound image and can either be manually selected by the user orautomatically selected, as discussed above in the Composite ImageEmbodiments section. Also as discussed in the Composite ImageEmbodiments section, multiple regions of interest can be selected.

Next, motion within the region of interest in measured (step 1020).Motion can be measured, between two image frames (i.e., image frame Nand image frame N+1). These two frames, which are initially held in theimage memory 980, are stored in the first and second image buffers 945,950, respectively. Images are preferably stored in the image memory 980and the frame buffers 945, 950 as 8-bit gray scale values at full pixeldensity.

To measure motion, the motion estimator 955 performs an image trackingoperation to find estimated pixel motion of a selected sub-block ofpixels from image frame N to image frame N+1. To measure motion, themotion estimator 955, which preferably includes a L64720A motionestimator from LSI Logic, performs a minimum-sum-of-absolute differencesoperation, as is well known in the art. Alternatively, a high power,programmed digital signal processing circuit, such as a TMS320C80circuit by Texas Instruments, can be used.

The image sub-block selector 940 determines which sub-blocks of theimage frame will be sent to the motion estimator 955. A sub-block maybe, for example, an 8×8, 16×16, 32×32, or 48×48 block of pixels from a512×512 pixel frame. It is presently preferred that a 32×32 or a 16×16block of pixels be used. Larger pixel blocks (such as a 32×32 block) mayrequire multiple motion estimators to be cascaded. The controller 915repeats the motion estimation process for all the sub-blocks in imageframe N, thereby generating motion estimates over the entire ultrasoundimage. Because motion estimates for adjacent sub-blocks are typicallysimilar, the motion smoother 960 can create the appearance of smoothsub-block motion by averaging the motion estimates.

Lastly, based on the measured motion, a motion-compensated interpolatedimage is generated and inserted between real ultrasound-image framesinside the region of interest (step 1030). In this step, the motionscaler 965 scales the motion estimate according to the number ofinterpolated frames that need to be generated. For example, if a userdesires to double the apparent frame rate, one interpolated frame willbe generated for insertion between image N and image N+1, and thescaling factor will be 0.5. If three frames are being inserted, thescaling factors of 0.25, 0.5, and 0.75 are used for successive motioninterpolation steps. The image interpolator 970 takes the selectedsub-block data from image frame N in the first image buffer 945 andapplies the scaled motion. That is, the image interpolator 970 changesthe pixel address for every pixel in the sub-block by the requiredamount and accumulates the result. This process is then repeated for allsub-blocks in the image frame. The interpolated frame can then beinserted between the two real frames, as is known in the art, andpresented to the user on the display 985.

As an example, consider the situation in which one interpolated frame isto be generated and the motion estimator 955 determines that a 32×32sub-block of pixels (initially located at locations (0-31, 0-31) inimage frame N) moved 8 pixels to the right and 8 pixels down. Becauseonly one frame will be inserted, the factor used by the motion scaler965 is 0.5. Accordingly, the scaled motion estimate is 4 pixels (0.5*8)to the right and 4 pixels (0.5*8) down. Thus, the pixel sub-block willbe placed at locations (4-35, 4-35) in the interpolated frame. Asdiscussed further below, it is also possible to add a component due tothe N+1 frame. In this case, the required motion is 4 pixels to the leftand 4 pixels up, so that, theoretically, the interpolated frame N andframe N+1 are superimposed.

By moving sub-blocks in this manner, it is possible that repositionedsub-blocks will overlap in the interpolated image frame. To deal withthe overlap problem, the pixel data in the region of overlap can beaveraged or the data from one of the sub-blocks can be given priority.

Another problem which can arise when sub-blocks are moved is that therecan be pixel locations in the interpolated image frame that do notcontain new pixel data (i.e., "holes" are formed in the interpolatedimage frame). One approach to dealing with holes is to eliminate thepossibility of their creation. That is, sub-blocks can be written over acopy of a real image frame (e.g., image frame N or image frame N+1). Inthis way, there will always be pixel information at every pixel locationof the interpolated image frame even if some locations in theinterpolated image frame do not contain new data. Another approach is tofill the holes with pixel information interpolated from surroundingsub-blocks.

There are several advantages to using this method. First, this methodincreases the apparent frame rate of an ultrasound imaging system beyondthe limitations normally imposed by, for example, line density, transittime, and Doppler processing delays. Because there is considerableredundancy between consecutive frames, image quality in many cases willnot suffer as a result of inserting interpolated frames.

Second, unlike applying a conventional interpolation filter, this methodtakes account of frame-to-frame motion and does not result in blurredimages when the target moves. As a result, motion-compensatedinterpolated images can be used in applications in which a high framerate is important.

Another advantage is that this method permits the use of lower channelcable counts in situations where high channel counts would otherwise berequired. For example, phase-aberration corrected images may requirevery high physical transducer element counts in 1.5 or 2 dimensionalarrays. Since cable costs (and termination costs in particular) can beprohibitively expensive, a potential option is to multiplex, forexample, half the array in one firing and the other half in the nextfiring. This multiplexing can be performed on a line-by-line basisrather than a frame-by-frame basis and is preferably performed by usingmultiples of the HV2xx family of multiplexers from Supertex, Inc.(Sunnyvale, Calif.) or other suitable circuitry. With this multiplexing,not only is the cable channel count halved, but the frame rate is alsohalved. By using the method of this preferred embodiment, full apparentframe rate can be achieved even with using lower channel cable counts.Additionally, this method can be used in a three-dimensional volume scanfrom a two-dimensional array where frame rate will be very slow.

Lastly, this method is easily implemented since image frame motionanalysis is largely a subset of the video operations (e.g.,Moving-Picture-Experts-Group (MPEG) operations) which are alreadyavailable in chip set solutions from multiple vendors, such as LSI Logicor C-Cube, for real-time motion-detection operations. Such motionanalysis is described in more detail below.

While the above method has been described in terms of scan-convertedimages, the motion estimations can be applied to raw acoustic line data(envelope detected, RF, or baseband In phase/Quadrature data), using theknown geometry of the lines (e.g., polar) to convert to required pixeltranslations (i.e., Cartesian). However, the method described above ispreferred on the grounds of simplicity.

As an alternative to the above, a real frame (instead of an interpolatedframe) can be acquired when an inaccurate, interpolated frame isgenerated. As described below, motion estimators that are compatiblewith MPEG compression standards typically supply pixel error signals. Ifthe correlation of one frame block to the next is poor, the sum of theerror signals will be large, indicating that the interpolated frame isinaccurate. To prevent the display of an inaccurate frame, if the sum ofthe error signals exceeds some preset or adjustable threshold level, theultrasound system 910 can acquire additional real images inside theregion of interest instead of generating an interpolated frame.

As another alternative, when the region of interest comprises less thanthe entire image and when motion-compensated interpolated frames areused inside the region of interest, image data outside the region ofinterest can be repeated to match the frame rate of the image inside theregion of interest.

ADAPTIVE MOTION-SENSING EMBODIMENTS

The system 1110 illustrated in FIG. 11 has a configuration similar tothat of the motion compensation embodiments and additionally comprises amotion sensor 1133. The motion sensor 1133, which is responsive tomovement of a housing of the transducer array 1135 and is coupled to thecontroller 1115, measures motion of the transducer probe. The controller1115 of this preferred embodiment comprises means for altering systemparameters in response to the presence or absence of detected motion.Specifically, the controller 1115 comprises means for automaticallyapplying certain imaging parameters within a region of interest of anultrasound-image frame in response to detected image or transducermotion and means for automatically altering the operating mode of thetransducer array 1135 in response to an absence of detected transducermotion. These two controller 1115 functions, which can be performed inreal time, will be described in more detail below.

Automatically Applying Imaging Parameters

As mentioned above, many users fail to optimize imaging parameters afterimage or transducer motion because of the complexity of the requiredadjustment. This imaging system 1110 solves that problem byautomatically applying certain imaging parameters in a region ofinterest in an ultrasound image in response to detected transducer orimage motion. As illustrated in FIG. 12, first, image or transducermotion is measured (step 1210). Then, the controller 1115 automaticallyapplies certain imaging parameters in at least one portion of anultrasound image in response to the measured motion (step 1220).

The first step in the method of this preferred embodiment is to measuretransducer or image motion (step 1210). Preferably, the motion estimator1155 measures motion of a sub-block of pixels between twoultrasound-image frames, as described in the Motion CompensationEmbodiments section and below. This measured motion is indicative oftransducer motion and image motion. Motion estimates can be taken in oneor more portions of the image frame. If motion estimates are taken inmore than one portion of the image, the controller 1115 preferably basesthe decision of whether to varying the imaging parameters on whether themeasured motion of a majority of the portions exceeds a threshold. Moreweight can be attached to the motion estimates derived in the middle ofthe image rather than at the sides since the user is usually mostinterested in that section of the image frame.

Transducer motion can also be measured by a motion sensor 1133 in thehousing of the transducer array 1135. The motion sensor 1133 can be anaccelerometer or a magnetic position sensor, such as a "Flock of Birds"magnetic position sensor from Ascension Technology Corporation(Burlington, Vermont).

In response to the measured motion, the controller 1115 automaticallyapplies certain imaging parameters in a region of interest in theultrasound-image frame to optimize system performance (step 1220). Asdescribed in the first two preferred embodiments above, a region ofinterest can comprise all or part of the frame and can be manually orautomatically selected. It is preferred that a single region of interestautomatically selected to cover all of the image frame be used to ensurethat imaging parameters are applied over the entire image frame inresponse to measured motion. Additionally, more than one region ofinterest can be selected.

In operation, if the measured motion exceeds a predetermined threshold,the controller 1115 automatically switches to a higher frame rate/lowerspatial resolution mode. In such a mode, imaging parameters areautomatically applied in the region of interest to improve temporalresolution to avoid image blurring. If the measured motion is below apredetermined threshold, the controller 1115 automatically switches to alower frame rate/higher spatial resolution mode. In this mode, imagingparameters are automatically applied in the region of interest toimprove spatial resolution. The Composite Image Embodiments sectiondiscusses a number of imaging parameters that can be applied to improvespatial or temporal resolution. Specifically, these parameters include:line density, number of transmit foci per scan line, pre-detectionfilter characteristics, post-detection filter characteristics,post-processing maps, ultrasound operating frequency, transmit power,logarithmic compression profiles, numbers of multiple receive lines pertransmit line, transmit pulse shape, receive frequency band, andpersistence.

For example, if the image exhibits signs of fast motion (e.g., the useris performing a sweeping search across the body or a tissue is quicklymoving), the controller 1115 automatically applies parameterscorresponding to a higher speed/lower resolution mode in order to takeaccount of the movement, thereby avoiding the presentation of a blurredmoving image. Conversely, if the image exhibits signs of very littlemotion (e.g., the user is concentrating on a region of great interest ora tissue that is relatively still), the controller 1115 automaticallyapplies parameters corresponding to a lower frame rate/higher spatialresolution mode to provide a more spatially-detailed display.

If the controller 1115 is frequently toggling between higher and lowerspatial resolution modes, the displayed image can be difficult to view.Accordingly, it is preferred that the controller 1115 take motionestimates over a sequence of image frames and compare the average ofthese estimates to a threshold value. In this way, the controller 1115automatically switches imaging parameters after detecting a sequence offrames of high motion, for example, instead of after detecting the firstframe of high motion. It is also preferred that the controller 1115 havean option to disable operation of this method, especially when atransducer probe is mostly stationary but occasional rapid motions areanticipated.

As discussed above, when there is little detected motion, the controller1115 automatically decreases frame acquisition and increases spatialresolution. To increase the apparent frame rate when the actual framerate is decreased, the methods of the Motion Compensation Embodimentssection can be used to insert motion-compensated interpolated framesbetween real frames.

Automatically Altering the Operating Mode of the Transducer Array

The controller 1115 of this system 1110 can additionally comprise meansfor automatically altering the operation mode of the transducer array1135 in response to an absence of detected image motion. As mentionedabove, the motion estimator 1160 can identify motion in the image,typically by measuring motion of a sub-block of pixels between twoultrasound-image frames using, for example, aminimum-sum-of-absolute-differences operation. Zero motion betweensuccessive frames indicates that the probe containing the transducerarray 1135 is not in use.

If the controller 1115 does not receive signals indicative of transducermotion for a given amount of time (e.g., one minute), the controller1115 alters the operating mode of the transducer array 1135. Forexample, the controller 1115 can completely remove power from thetransducer array 1135 or can place the transducer array 1135 in a"sleep" mode--an imaging mode using less power. For example, during"sleep" mode, the controller 115 can disable only the imaging modes thatcause elevated temperatures in the transducer array 1135. Alternatively,during the "sleep" mode, the system 1115 can acquire real image data ata very low rate (e.g., once per second) until motion is detected. Oncemotion is detected, full power can be restored to the transducer array1135, and the pre-"sleep" operating mode of the transducer array 1135can be resumed. Additionally, a user can exit the "sleep" mode manuallyby entering an input to the user interface 1120 (e.g., striking a key ona keyboard).

The controller 1115 can alert a user that the operating mode of thetransducer array 1135 has been altered by, for example, displaying amessage on the display 1185. The controller 1115 can also synchronizethe system 1110 to ensure that the motion estimator 1155 is analyzingthe appropriate image frames (i.e., ensuring that image repeateddisplayed in a "freeze frame" operation are not analyzed for motion).

Since the system 1110 is responsive to any motion, it is not necessaryto search for two-dimensional motion. Single acoustic line data can beused to perform one-dimensional correlations between successivelyacquired lines to determine if the position of peak cross-correlation isnon-zero and/or if the peak correlation value is high (as would be thecase when there is no image motion). The position of thecross-correlation repeatedly being zero indicates that no probe motionhas occurred. This method of detecting motion is much lesscomputationally intensive than methods that compare pixels on apoint-to-point basis over the image.

By using the method and system of this preferred embodiment, the problemof transducer overheating is avoided. Unlike past solutions to theproblem, using ultrasound-image frame information to measure transducermotion is inexpensive and does not require modifications to be made tothe probe housing the transducer array or user intervention.Additionally, because motion estimates are used, this method can be muchless computationally intensive than methods that compare pixels on apoint-to-point basis over the image. Also, if a system is calculatingmotion estimates for other functions (for example, for generatingmotion-compensated interpolated frames), those motion estimates can beused to detect transducer motion, further making this technique lesscomputationally intensive than techniques that use point-to-pointcomparisons.

DISTORTION CORRECTING EMBODIMENTS

FIG. 13 is a block diagram of an ultrasound imaging system 1310. Thissystem 1310 is configured similarly to that of the system 1110 in theAdaptive Motion-Sensing Embodiments section. This system 1310additionally comprises an acoustic line memory 1312 responsive to thecontroller 1315 and coupled to the scan converter 1375. The controller1315 comprises means for generating a distortion-corrected image insidea region of interest in response to measured image or transducer motion.Specifically, the controller 1315 comprises means for reprocessing linedata with corrected line spacing and means for repositioning sub-blocksof pixels in response to detected transducer or image motion. Thesefunctions, which can be performed in real time, are described below andin reference to FIG. 14.

FIG. 14 is a flow chart of a method for generating adistortion-corrected image inside a region of interest in response tomeasured image or transducer motion. First, transducer or image motionis measured (step 1410). In response to the measured motion, thecontroller 1315 can estimate the effect of the measured motion on linespacing and automatically reprocess line data with corrected linespacing (step 1420). Additionally, the controller 1315 can repositionsub-blocks of pixels in the ultrasound-image frame in response tomeasured motion (step 1430).

As shown in FIG. 14, first transducer or image motion is measured (step1410). As described in the above-preferred embodiments, it is preferredthat the motion estimator 1355 measure motion of a sub-block of pixelsbetween two ultrasound-image frames in a region of interest. Asdescribed in the first two preferred embodiments above, a region ofinterest can comprise all or part of the frame and can be manually orautomatically selected. Additionally, more than one region of interestcan be selected. Alternatively, transducer motion can also be measuredby a motion sensor 1333 in the housing of the transducer array 1335. Themotion sensor 1333 can be an accelerometer or a magnetic positionsensor, such as a "Flock of Birds" magnetic position sensor fromAscension Technology Corporation (Burlington, Vermont).

Next, in response to the measured motion, the controller 1315 canestimate the effect of the measured motion on line spacing andautomatically reprocess line data with corrected line spacing (step1420). For example, suppose that an ultrasound image frame is comprisedof 100 lines and that the lines are physically spaced 0.5 mm apart onthe surface of the transducer array 1335. The width of the image frameis, therefore, 50 mm. Also suppose that a single image frame takes 0.25seconds to acquire and that the detected transducer motion is 4 lines(i.e., 2 mm) to the right between successive frames. As a result of thismotion, the controller 1315 estimates that the right-most fired scanline will correspond to a location that is 2 mm outside of the imageframe. The resulting image frame is distorted in the sense that 2 mm ofinformation are not displayed in the 50 mm-wide image frame.

To compensate for this distortion, the display width of the image framemust be expanded by 2 mm and the line data (which is stored in theacoustic line memory 1312) must be reprocessed accordingly. Toaccomplish the reprocessing, the controller 1315 reprocesses the linedata in the scan converter 1375 by changing the line spacing of theacoustic lines from 0.5 mm to 0.52 mm. A similar process can be appliedto distortions in the depth direction. The scan-converted,distortion-corrected image can then be displayed.

Another type of image motion-related distortion is caused by time delaybetween lines acquired on the left and right hand side of the imageframe. The controller 1315 can automatically reposition sub-blocks ofpixels in the ultrasound-image frame (step 1430). As an example, supposesub-block P (which in this example is a single pixel) is located atposition X1 in the first acquired frame (FIG. 15) and at position X2 inthe second acquired frame (FIG. 16). By a scan line's lateral position,the controller 1315 can calculate when pixels were acquired in a frame.In this example, in the first acquired frame, the left-most scan linewas acquired at time T0, sub-block P was acquired at time T1, and theright-most scan line was acquired at time T2. In the second acquiredframe, the left-most scan line was acquired at time T3, sub-block P wasacquired at time T4, and the right-most scan line was acquired at timeT5.

Because sub-block P is moving in this example, its location at time T2will be different from its location at time T1. Accordingly, if ascan-converted image is displayed at time T2 without taking into accountsub-block P's movement, the displayed image will contain inaccurateinformation regarding sub-block P's position.

To correct for this distortion, the velocity of sub-block P iscalculated based on the time that the system 1310 acquires the scan linethat captured sub-block P, not on the time that the system acquired theright-most scan line. Thus, the velocity of sub-block P would be(X2-X1)/(T4-T1). Accordingly, the position of sub-block P at a time whenthe last scan line of the frame is acquired (i.e., T2) would be X1+(X2-X1)/(T4-T1)!* T2-T1!. With this information, the controller 1315instructs the scan converter 1375 to reposition sub-block P and displaya corrected frame rather than the originally acquired, distorted frame.

When the previous image frame was corrected using this method, it can beused as the basis for the next calculation, thereby producing anundistorted sequence of images.

Additionally, this method can be used with the methods of the MotionCompensation Embodiments section to provide a more exact calculation ofa sub-block location in a motion-compensated interpolated frame. In themethods of the Motion Compensation Embodiments section, it can beassumed for simplicity that all pixels in the real frames were acquiredat the time that the right-most scan line was acquired. With thisassumption, in a motion-compensated interpolated frame generated betweenthe frames of FIGS. 15 and 16, the velocity of sub-block P would be(X2-X1)/(T5-T2). If one interpolated frame were generated at a timehalf-way between the times that the right-most lines of the two frameswere acquired, the position of sub-block P in the interpolated framewould be X1+ (X2-X1)/(T5-T2)!* (T5-T2)/2!, or (X2+X1)/2. If sub-block Por the transducer moved during the time the frame was scanned, thiscalculated position of sub-block P would result in a distortedinterpolated frame, as described above.

The method of this preferred embodiment can correct for this distortion.That is, the velocity of sub-block P can be based on the times when thescan lines acquired sub-block P (i.e., times T1 and T4, not times T2 andT5). Accordingly, the position of sub-block P in the interpolated framewould be X1+ (X2-X1)/(T4-T1)!* (T5-T2)/2!.

MOTION ESTIMATION

In the preferred embodiments described above, transducer or image motionis measured. The general field of motion measurement between successiveimage frames has been discussed widely in the public literature (seeImage Sequence Analysis, T. S. Huang (Editor) Springer-Verlag, 1981 and"Interframe Interpolation of Cinematic Sequences," Ribas-Corbera &Sklansky, Journal of Visual Communication and Image Representation, Vol.4, No. 4, December 1993 (pages 392-46)). While any means of motionmeasurement can be used, it is presently preferred that a motionestimator employing a Moving-Picture-Experts-Group (MPEG) standard beused to track the movement of a block of pixels between image frames.While MPEG applies to both video and audio, only video (specifically,the luminance component) is relevant here. It is preferred that aL64720A motion estimator from LSI Logic be used.

An MPEG motion estimator calculates, in real time, motion vectors for ablocks of pixels moving from one image frame to the next. It ispreferred that the ultrasound image be split into 32×32 or 16×16macroblocks. Between successive input frames, a best fitting motionestimation is made for each block, preferably using a sum of absolutedifferences in pixel intensity. Other techniques can also be used. Anerror between the two blocks corresponding to the degree of correlationcan also be calculated. The vector shifts and error components are thencoded for transfer.

With the motion vectors, the ultrasound system can generate aninterpolated frame from: (a) forward prediction from the earlier frame,(b) backward prediction from the later image, or (c) a combination ofthe forward and backward predictions. If a motion estimate isquestionable (i.e., the calculated error is large), it can be replacedwith a mean of surrounding low-error estimates. Alternatively the matrixof vector estimates may be low-pass or median filtered. Other filteringoperations, including non-linear filtering, can also be used. In thecase of a translational motion, it is known that all estimates of motionshould be similar and therefore a high degree of low-pass filtering ispermissible without interfering with image quality.

To obtain a smoother image-motion effect, the ultrasound system caninterpolate intermediate displacement vectors. For example, if theultrasound system computes motion estimates for each of the 16×16 blocksof pixels in the image frame, it can generate interpolated displacementvectors to effectively cover the image frame with 8×8 blocks of pixels.This process may be repeated with smaller blocks (4×4, etc.) to obtain asmoother motion effect. Notice that larger blocks are preferred becausethey will generally be more stable and accurate. Also, larger blocks cantolerate larger pixel displacements. Since processing large blocks iscomputationally expensive, multiple processors can be used.Specifically, the TMS320C80 circuit from Texas Instruments allows forparallel processing.

If image motions are very large or computational complexities imposelimitations on maximum block size, the original image data can be downsampled in order to increase real pixel spacing. For example, pixelsub-blocks can be averaged and replaced by single values. Other methods,preferably ones in which a low-pass filter is used prior to downsampling, can be used and are well known in the art. Additionally, pixelvalue resolution can be compressed to lessen computation complexity.

Similarly, the output-pixel-value-to-input-signal-level mapping can bealtered to give more reliable motion estimates. For example, if theregion is mostly white (i.e., it requires high pixel values), it may bepreferable to lower the mean pixel value and disperse it over theavailable range (e.g., if the pixel range is 128-255, it may be remappedto 0-255 with increments of 2 by subtracting 128 from the originalvalues and doubling for more effective use of the availablecomputational resource).

As a result of motion-compensated interpolation, the interpolated imagemay contain artifacts. Because interpolated images are presented in amoving sequence of frames, small, individual artifacts may not be verynoticeable. Regularly occurring or large, blocky artifacts may bediscernible, however. To remove such artifacts, a low-pass filter can beapplied to the entire interpolated image or in regions of the blockboundaries, e.g., +/- four pixels from the boundary. A spatialtwo-dimensional low-pass filter applied to the image may benon-symmetric. Since axial resolution is generally better than lateralresolution, the low-pass cutoff frequency in the lateral (width)direction may be lower than in the axial (depth) direction.

Another way to remove artifacts is by offsetting the centers of theblocks that cover the image frame in random directions to reduce thelikelihood of a regular block structure being evident. With such anapproach, it is important to avoid holes by ensuring that the entireimage frame is covered. The centers can also be offset so that theblocks overlap (e.g., 16×16 blocks spaced at 12 pixel spacing). Whereblocks overlap, the pixel output level is preferably given by the meanof the contributing pixel values.

Additionally, because artifacts are more noticeable in a freeze frame orslow motion replay mode, the ultrasound system can be modified topresent only real images in these modes.

As mentioned above, using a motion estimator employing an MPEG standardis merely one may in which motion can be measured. One skilled in theart can derive several enhancements including, for example, usinggradient or differential techniques (see Image Sequence Analysis byHuang), using optical flow based methods (see "Restoration of theVelocity Field of the Heart from Two-Dimensional Echocardograms" byMailloux et al., IEEE Transactions on Medical Imaging, Vol. 8, No. 2,June 1989 (pages 143-153)), and using the well-known technique ofsub-pixel resolution motion tracking. See also "A Locally QuadraticModel of the Motion Estimation Error Criterion Function and ItsApplication to Subpixel Interpolation," Li and Gonzales, IEEETransactions on Circuits and Systems for Video Technology, Vol. 6,No. 1. Page 188, February 1996.

One skilled in the art can also derive alternative means of detectingmotion. A general purpose, high-powered digital-signal-processorintegrated circuit can be programmed with high-level code to performboth MPEG compression and decompression. It is preferred that aTMS320C80 MVP circuit and high-level code, both available from TexasInstruments, be used. One skilled in the art would be able to adapt thecode to implement the functions described in the above preferredembodiments. Because they are less specialized than circuits employingthe MPEG standard, modified general-purpose circuits have thedisadvantage of being less efficient than MPEG motion processors.

It is preferred to perform motion estimates on a scan-converted,rectangular acoustic grid. With such a grid, each pixel has only alimited but necessary level of detail (e.g., 8 bits). If the level ofdetail is too great (e.g,. 16 or 24 bits) or if the pixel density of theoutput display device (which contains multiple pixels per acoustic grid)is used, computations become more complex.

In addition to scan-converted, rectangular acoustic grids, motionestimates can be applied to RF (raw acoustic line) data. In this contextraw acoustic line data means any of true RF, IF, or baseband Inphase/Quadrature data. Motion estimates on RF data for blood speckletracking are known in the art (see "A Novel Method for Angle IndependentUltrasonic Imaging of Blood Flow and Tissue Motion," L. N. Bohs, IEEETrans, BME Vol. 38, No. 3, March 1991, pp. 280-286). If RF data framesare used to produce an interpolated "raw acoustic line data frame,"distortions due to the fact that the lines are not in a rectangularformat may cause only minor problems. However, it should be noted thatin the case of sector- or Vector®-type format, motions appear to belarger at the top than at the bottom due to the high line density at thetop of the image. For example, for a given azimuthal translation, fivelines are crossed at the top of the image, while only three lines arecrossed at the bottom of the image. Accordingly, if the RF image data ismotion estimated, it is preferred that the data is sampled at leasttwice per RF period.

It is important to note that the various features described in the aboveembodiments can be used alone or in various sub-combinations with oneanother. It is also important to note that the foregoing detaileddescription is merely an illustration of selected forms that theinvention can take and not as a definition of the invention. It is onlythe following claims, including all equivalents, that are intended todefine the scope of this invention.

What is claimed is:
 1. In an ultrasound imaging system comprising atransducer array, a transmit beamformer, and a receive beamformer, theimprovement comprising:selecting means for selecting a region ofinterest within an ultrasound-image frame, the region of interestcomprising less than an entire ultrasound-image frame; means forselectively applying a first set of imaging parameters inside the regionof interest, said first set being different from a second set of imagingparameters applied outside the region of interest; and image assemblymeans for assembling a composite image comprising a first image portionwithin the region of interest and a second image portion outside theregion of interest, said first and second image portions being in thesame imaging mode and said image assembly means being responsive to theselecting means, the means for selectively applying, and the receivebeamformer.
 2. The invention of claim 1, wherein the selecting meanscomprises motion detection means for automatically selecting the regionof interest in response to detected motion.
 3. The invention of claim 1,wherein the selecting means comprises a user interface means formanually selecting the region of interest.
 4. The invention of claim 1,wherein the means for selectively applying comprises means for applyinga different set of imaging parameters inside the region of interest thanoutside the region of interest to improve spatial resolution within theregion of interest.
 5. The invention of claim 4, wherein the means forselectively applying comprises means for applying a different linedensity inside the region of interest than outside the region ofinterest.
 6. The invention of claim 4, wherein the means for selectivelyapplying comprises means for applying a different number of receive lineper transmit line inside the region of interest than outside the regionof interest.
 7. The invention of claim 4, wherein the means forselectively applying comprises means for applying a different transmitfoci per scan line inside the region of interest than outside the regionof interest.
 8. The invention of claim 4, wherein the means for varyingimaging parameters comprises means for applying different pre-detectionfilter characteristics inside the region of interest than outside theregion of interest.
 9. The invention of claim 4, wherein the means forvarying imaging parameters comprises means for applying differentpost-detection filter characteristics inside the region of interest thanoutside the region of interest.
 10. The invention of claim 4, whereinthe means for varying imaging parameters comprises means for applyingdifferent post-processing map characteristics inside the region ofinterest than outside the region of interest.
 11. The invention of claim4, wherein the means for varying imaging parameters comprises means forapplying a different ultrasound operating frequency inside the region ofinterest than outside the region of interest.
 12. The invention of claim4, wherein the means for varying imaging parameters comprises means forapplying a different transmit power inside the region of interest thanoutside the region of interest.
 13. The invention of claim 4, whereinthe means for varying imaging parameters comprises means for applying adifferent logarithmic compression profile inside the region of interestthan outside the region of interest.
 14. The invention of claim 4,wherein the means for varying imaging parameters comprises means forapplying a different transmit pulse shape inside the region of interestthan outside the region of interest.
 15. The invention of claim 4,wherein the means for varying imaging parameters comprises means forreceiving a different frequency band inside the region of interest thanoutside the region of interest.
 16. The invention of claim 4, whereinthe means for varying imaging parameters comprises means for receiving aharmonic frequency band inside the region of interest and receiving afundamental frequency band outside the region of interest.
 17. Theinvention of claim 1, wherein the means for selectively applyingcomprises means for applying a different set of imaging parametersinside the region of interest than outside the region of interest toimprove temporal resolution within the region of interest.
 18. Theinvention of claim 17, wherein the means for varying imaging parameterscomprises means for increasing an actual frame rate inside the region ofinterest by acquiring more real ultrasound-image frames inside theregion of interest than outside the region of interest.
 19. Theinvention of claim 17, wherein the means for varying imaging parameterscomprises means for applying a different persistence level inside theregion of interest than outside the region of interest.
 20. Theinvention of claim 1, wherein the image assembly means comprises meansfor creating a spatially smooth transition between the first imageportion and the second image portion.
 21. The invention of claim 1,wherein the image assembly means comprises means for matching a framerate of the first image portion with a frame rate of the second imageportion.
 22. The invention of claim 1, wherein said first and secondimage portions are based on substantially the same receive frequencyband.
 23. In an ultrasound imaging system comprising a transducer arrayand a transmit beamformer, the improvement comprising:a first signalpath for processing a first ultrasound-image portion; a second signalpath for processing a second ultrasound-image portion; image combinationlogic comprising means for assembling a composite image comprising thefirst image portion and the second image portion, said first and secondimage portions being in the same imaging mode and said image assemblerbeing responsive to the first and second signal paths; and a controller,coupled to the transmit beamformer, the first signal path, the secondsignal path, and the image assembler, comprising means for selectivelyapplying a first set of imaging parameters to the first ultrasound-imageportion and a second set of imaging parameters to the secondultrasound-image portion, said first set being different from saidsecond set.
 24. The invention of claim 23, wherein the first signal pathcomprises a first processor, a first line memory, and a beamformer andwherein the second signal path comprises a second processor, a secondline memory, and a beamformer.
 25. The invention of claim 23, whereinthe first signal path comprises a first processor, a first scanconverter, and a beamformer and wherein the second signal path comprisesa second processor, a second scan converter, and a beamformer.
 26. Theinvention of claim 23, wherein said first and second image portions arebased on substantially the same receive frequency band.
 27. In anultrasound imaging system comprising a transducer array, a transmitbeamformer, and a receive beamformer, the improvement comprising:acontroller coupled to the transmit beamformer and the receivebeamformer; and a processor responsive to the controller, saidcontroller and processor comprising means for selectively applying afirst set of imaging parameters to a first image portion, said first setbeing different from a second set of imaging parameters applied to asecond image portion, said first and second image portions being in thesame imaging mode.
 28. The invention of claim 27, wherein said first andsecond image portions are based on substantially the same receivefrequency band.
 29. A method for improving spatial characteristicswithin a region of interest in an ultrasound image comprising the stepsof:(a) selecting a region of interest of an ultrasound image, the regionof interest comprising less than an entire ultrasound image; then (b)selectively applying a first set of imaging parameters inside the regionof interest to improve spatial resolution inside the region of interest,said first set being different from a second set of imaging parametersapplied outside the region of interest; and (c) assembling a compositeimage comprising a first image portion within the region of interest anda second image portion outside the region of interest, said first andsecond image portions being in the same imaging mode.
 30. The method ofclaim 29, wherein the region of interest in step (a) is manuallyselected by a user.
 31. The method of claim 29, wherein the region ofinterest in step (a) is automatically selected in response to measuredmotion.
 32. The method of claim 29, wherein step (b) is manuallyperformed.
 33. The method of claim 29, wherein step (b) is automaticallyperformed.
 34. The method of claim 29, wherein step (b) comprisesapplying a different line density inside the region of interest thanoutside the region of interest.
 35. The method of claim 29, wherein step(b) comprises applying a different number of receive line per transmitline inside the region of interest than outside the region of interest.36. The method of claim 29, wherein step (b) comprises applying adifferent transmit foci per scan line inside the region of interest thanoutside the region of interest.
 37. The method of claim 29, wherein step(b) comprises applying different predetection filter characteristicsinside the region of interest than outside the region of interest. 38.The method of claim 29, wherein step (b) comprises applying differentpostdetection filter characteristics inside the region of interest thanoutside the region of interest.
 39. The method of claim 29, wherein step(b) comprises applying different postprocessing map characteristicsinside the region of interest than outside the region of interest. 40.The method of claim 29, wherein step (b) comprises applying a differentultrasound operating frequency inside the region of interest thanoutside the region of interest.
 41. The method of claim 29, wherein step(b) comprises applying a different transmit power inside the region ofinterest than outside the region of interest.
 42. The method of claim29, wherein step (b) comprises applying a different logarithmiccompression profile inside the region of interest than outside theregion of interest.
 43. The method of claim 29, wherein step (b)comprises applying a different transmit pulse shape inside the region ofinterest than outside the region of interest.
 44. The method of claim29, wherein step (b) comprises receiving a different frequency bandinside the region of interest than outside the region of interest. 45.The method of claim 29, wherein step (b) comprises receiving a harmonicfrequency band inside the region of interest and receiving a fundamentalfrequency band outside the region of interest.
 46. The method of claim29, wherein step (c) comprises using motion-compensated interframeinterpolation to match a frame rate of the first image portion withinthe region of interest with a frame rate of the second image portionoutside the region of interest.
 47. The method of claim 29, wherein step(c) further comprises the step of creating a spatially smooth transitionbetween the first image portion within the region of interest and thesecond image portion outside the region of interest.
 48. The method ofclaim 29 further comprising the step of repeating steps (a)-(c) for aplurality of regions of interest.
 49. The method of claim 29, whereinsteps (a)-(c) are performed in real time.
 50. The method of claim 29,wherein said first and second image portions are based on substantiallythe same receive frequency band.
 51. A method for improving temporalcharacteristics within a region of interest of an ultrasound imagecomprising the steps of:(a) selecting a region of interest of anultrasound image, the region of interest comprising less than an entireultrasound image; then (b) selectively applying a first set of imagingparameters inside the region of interest to improve temporal resolutioninside the region of interest, said first set being different from asecond set of imaging parameters applied outside the region of interest;and (c) assembling a composite image comprising a first image portionwithin the region of interest and a second image portion outside theregion of interest, said first and second image portions being in thesame imaging mode.
 52. The method of claim 51, wherein the region ofinterest in step (a) is manually selected by a user.
 53. The method ofclaim 51, wherein the region of interest in step (a) is automaticallyselected in response to measured motion.
 54. The method of claim 51,wherein step (b) is manually performed.
 55. The method of claim 51,wherein step (b) is automatically performed.
 56. The method of claim 51,wherein step (b) comprises increasing an actual frame rate inside theregion of interest by acquiring more real ultrasound-image frames insidethe region of interest than outside the region of interest.
 57. Themethod of claim 51, wherein step (b) comprises applying a differentpersistence level inside the region of interest than outside the regionof interest.
 58. The method of claim 57, wherein persistence inside theregion of interest is varied as a function of measured motion within theregion of interest.
 59. The method of claim 58, wherein motion estimatesof a sub-block of moving pixels are used to measure motion.
 60. Themethod of claim 58, wherein a difference between pixels at a samespatial location in successive frames is used to measure motion.
 61. Themethod of claim 57, wherein step (b) comprises using motion-compensatedinterpolated frames to generate a persistence filtered image.
 62. Themethod of claim 57, wherein step (b) comprises using motion estimates ofa sub-block of pixels with real ultrasound image frames to generate apersistence filtered image.
 63. The method of claim 51, wherein step (c)comprises using motion-compensated interframe interpolation to match aframe rate of the first image portion within the region of interest witha frame rate of the second image portion outside the region of interest.64. The method of claim 51, wherein step (c) further comprises the stepof creating a smooth transition between the first image portion withinthe region of interest and the second image portion outside the regionof interest.
 65. The method of claim 51 further comprising the step ofrepeating steps (a)-(c) for a plurality of regions of interest.
 66. Themethod of claim 51, wherein steps (a)-(c) are performed in real time.67. A method for generating a persistence filter coefficient of a regionin an ultrasound-image frame comprising the steps of:(a) measuring imagemotion in an area of a region using at least two ultrasound-imageframes; and then (b) applying a persistence filter coefficient for theregion as a function of the measured motion in step (a), said functioncomprising a relationship wherein the persistence filter coefficientincreases as the difference in pixel intensity decreases.
 68. The methodof claim 67, wherein image motion is measured in step (a) is calculatedby the equation: ##EQU2## wherein I(n,x,y) comprises an intensity valueof a pixel in a current ultrasound-image input frame, O(n-1,x,y)comprises an intensity value of a pixel in a previous ultrasound-imageoutput frame, and (x,y) in A(i,j) comprises a set of pixels in an areaof a region.
 69. The method of claim 67, wherein image motion ismeasured in step (a) is using motion estimates of a sub-block of movingpixels.
 70. The method of claim 67, wherein the area in step (a)comprises an entire region.
 71. The method of claim 67, wherein the areain step (a) comprises a portion of the region.
 72. The method of claim71, wherein the portion of the region comprises pixels having anintensity amplitude greater than a predetermined threshold.
 73. Themethod of claim 67 further comprising the step of:(c) increasing thepersistence filter coefficient in response to detected noise.
 74. Themethod of claim 67 further comprising the step of:(c) repeating steps(a) and (b) for a plurality of regions.
 75. The method of claim 73,wherein the plurality of regions covers an entire ultrasound imageframe.
 76. In an ultrasound imaging system comprising a transducerarray, a transmit beamformer, and a receive beamformer, the improvementcomprising:means, responsive to the receive beamformer, for measuringmotion within a sequence of at least two ultrasound-image frames in aregion of interest; and means, responsive to the receive beamformer, forgenerating a motion-compensated interpolated image inside the region ofinterest in response to measured motion.
 77. The invention of claim 76further comprising selecting means for selecting a region of interestwithin an ultrasound image, said selecting means being coupled to themeans for measuring.
 78. The invention of claim 77, wherein theselecting means comprises motion detection means for automaticallyselecting the region of interest in response to detected motion.
 79. Theinvention of claim 77, wherein the selecting means comprises a userinterface means for manually selecting the region of interest.
 80. Theinvention of claim 77, wherein the region of interest comprises lessthan an entire ultrasound image.
 81. The invention of claim 77, whereinthe region of interest comprises an entire ultrasound image.
 82. Theinvention of claim 76, wherein said means for measuring motion comprisesmeans for determining motion of a sub-block of pixels between a firstand second ultrasound image frame.
 83. The invention of claim 76,wherein said means for measuring motion comprises an image sub-blockselector responsive to the receive beamformer, a motion estimatorresponsive to the image sub-block selector, a motion scaler responsiveto the motion estimator, and an image interpolator responsive to themotion scaler.
 84. The invention of claim 83 further comprising a motionsmoother responsive to the motion estimator and coupled to the motionscaler.
 85. A method for increasing an apparent frame rate within aregion of interest in an ultrasound image comprising the steps of:(a)selecting a region of interest of an ultrasound image; then (b)measuring motion between a first and second ultrasound-image frame inthe region of interest; then (c) generating a motion-compensatedinterpolated image inside the region of interest based on the measuredmotion of step (b); and then (d) inserting the motion-compensatedinterpolated image between the first and second ultrasound-image framesinside the region of interest.
 86. The method of claim 85, wherein theregion of interest comprises less than an entire ultrasound image. 87.The method of claim 85, wherein the region of interest comprises anentire ultrasound image.
 88. The method of claim 85, wherein the regionof interest in step (a) is manually selected by a user.
 89. The methodof claim 85, wherein the region of interest in step (a) is automaticallyselected.
 90. The method of claim 85 further comprising the step of:(e)repeating steps (a) and (d) are repeated for a plurality of regions ofinterest.
 91. The method of claim 85, wherein step (b) comprises thesteps of:(b1) selecting a sub-block of pixels to be analyzed; and (b2)measuring motion of the sub-block of pixels between a first and secondultrasound-image frame in the region of interest.
 92. The method ofclaim 91 further comprising the step of:(b3) repeating steps (b1) and(b2) for a plurality of sub-blocks.
 93. The method of claim 92 furthercomprising the step of averaging the measured motion of at least twosub-blocks.
 94. The method of claim 85, wherein step (c) comprises thestep of averaging pixel data in a location in the motion-compensatedinterpolated image which comprises overlapping data.
 95. The method ofclaim 85, wherein step (c) comprises the step of writing new pixel dataover existing pixel data from a real ultrasound-image frame.
 96. Themethod of claim 85, wherein step (c) comprises the step of writinginterpolated pixel data in a location in the motion-compensatedinterpolated image free of new pixel data.
 97. The method of claim 85,wherein steps (c) and (d) are performed in response to a pixel errorsignal not exceeding a threshold level and wherein the method furthercomprises the step of:(e) acquiring at least one additional realultrasound-image frame inside the region of interest in response to apixel error signal exceeding a threshold level.
 98. The method of claim85, wherein steps (a)-(d) are performed in real time.
 99. A method forautomatically applying ultrasound imaging parameters in an ultrasoundimage in response to transducer or image motion comprising the stepsof:(a) measuring motion; and then (b) automatically applying a differentset of imaging parameters inside the region of interest than outside theregion of interest in response to the motion measured in step (a). 100.The method of claim 99, wherein motion is measured in step (a) by amotion sensor in a transducer.
 101. The method of claim 99, whereinmotion is measured in step (a) by measuring motion of a sub-block ofpixels between at least two ultrasound-image frames.
 102. The method ofclaim 101, wherein motion of a plurality of sub-blocks of pixels ismeasured.
 103. The method of claim 99, wherein the region of interest instep (b) comprises a portion of an entire ultrasound image.
 104. Themethod of claim 99, wherein the region of interest in step (b) comprisesan entire ultrasound image.
 105. The method of claim 99, wherein theregion of interest in step (b) is manually selected by a user.
 106. Themethod of claim 99, wherein the region of interest in step (b) isautomatically selected.
 107. The method of claim 99 further comprisingthe step of repeating step (b) for a plurality of regions of interest.108. The method of claim 99, wherein step (b) comprises the step ofautomatically applying a different set of imaging parameters inside theregion of interest than outside the region of interest to improvetemporal resolution within the region of interest in response tomeasured motion exceeding a threshold value.
 109. The method of claim99, wherein step (b) comprises the step of automatically applying adifferent set of imaging parameters inside the region of interest thanoutside the region of interest to improve spatial resolution within theregion of interest in response to measured motion being below athreshold value.
 110. The method of claim 99 further comprising the stepof generating a motion-compensated interpolated image inside the regionof interest based on the measured motion of step (a).
 111. The method ofclaim 99, wherein step (b) comprises the step of automatically applyinga different line density inside the region of interest than outside theregion of interest in response to measured motion.
 112. The method ofclaim 99, wherein step (b) comprises the step of automatically applyinga different transmit foci per scan line inside the region of interestthan outside the region of interest in response to measured motion. 113.The method of claim 99, wherein step (b) comprises the step ofautomatically applying different pre-detection filter characteristicsinside the region of interest than outside the region of interest inresponse to measured motion.
 114. The method of claim 99, wherein step(b) comprises the step of automatically applying differentpost-detection filter characteristics inside the region of interest thanoutside the region of interest in response to measured motion.
 115. Themethod of claim 99, wherein step (b) comprises the step of automaticallyapplying different post-processing map characteristics inside the regionof interest than outside the region of interest in response to measuredmotion.
 116. The method of claim 99, wherein step (b) comprises the stepof automatically applying a different ultrasound operating frequencyinside the region of interest than outside the region of interest inresponse to measured motion.
 117. The method of claim 99, wherein step(b) comprises the step of automatically applying a different transmitpower inside the region of interest than outside the region of interestin response to measured motion.
 118. The method of claim 99, whereinstep (b) comprises the step of automatically applying a differentlogarithmic compression profile inside the region of interest thanoutside the region of interest in response to measured motion.
 119. Themethod of claim 99, wherein step (b) comprises the step of automaticallyapplying a different number of receive line per transmit line inside theregion of interest than outside the region of interest in response tomeasured motion.
 120. The method of claim 99, wherein step (b) comprisesthe step of automatically applying a different transmit pulse shapeinside the region of interest than outside the region of interest inresponse to measured motion.
 121. The method of claim 99, wherein step(b) comprises the step of automatically receiving a different frequencyband inside the region of interest than outside the region of interestin response to measured motion.
 122. The method of claim 99, whereinstep (b) comprises the step of automatically receiving a harmonicfrequency band inside the region of interest and receiving a fundamentalfrequency band outside the region of interest in response to measuredmotion.
 123. The method of claim 99, wherein step (b) comprises the stepof automatically applying a different persistence level inside theregion of interest than outside the region of interest in response tomeasured motion.
 124. In an ultrasound imaging system comprising atransducer array, a transmit beamformer, and a receive beamformer, theimprovement comprising:means, responsive to the receive beamformer, formeasuring motion; and means, responsive to said means for measuringmotion, for automatically applying a different set of imaging parametersinside the region of interest than outside the region of interest inresponse to measured motion.
 125. The invention of claim 124 furthercomprising means for generating a motion-compensated interpolated imageinside the region of interest in response to measured motion, said meansfor generating being responsive to said means for measuring motion. 126.The invention of claim 124, wherein said means for measuring motioncomprises means for determining motion of a sub-block of pixels betweenat least two ultrasound image frames.
 127. The invention of claim 124,wherein said means for measuring motion comprises an image sub-blockselector responsive to the receive beamformer, a motion estimatorresponsive to the image sub-block selector, a motion scaler responsiveto the motion estimator, and an image interpolator responsive to themotion scaler.
 128. The invention of claim 127 further comprising amotion smoother responsive to the motion estimator and coupled to themotion scaler.
 129. The invention of claim 124, wherein said means formeasuring motion comprises a motion sensor responsive to the transducer.130. The invention of claim 124, wherein said means for automaticallyapplying comprises means for automatically applying a different set ofimaging parameters inside the region of interest than outside the regionof interest in response to measured motion exceeding a threshold valueto improve temporal resolution.
 131. The invention of claim 124, whereinsaid means for automatically applying comprises means for automaticallyapplying a different set of imaging parameters inside the region ofinterest than outside the region of interest in response to measuredmotion being below a threshold value to improve spatial resolution. 132.The invention of claim 124, wherein said means for automaticallyapplying comprises means for automatically applying a different linedensity inside the region of interest than outside the region ofinterest in response to measured motion.
 133. The invention of claim124, wherein said means for automatically applying comprises means forautomatically applying a different transmit foci per scan line insidethe region of interest than outside the region of interest in responseto measured motion.
 134. The invention of claim 124, wherein said meansfor automatically applying comprises means for automatically applyingdifferent pre-detection filter characteristics inside the region ofinterest than outside the region of interest in response to measuredmotion.
 135. The invention of claim 124, wherein said means forautomatically applying comprises means for automatically applyingdifferent post-detection filter characteristics inside the region ofinterest than outside the region of interest in response to measuredmotion.
 136. The invention of claim 124, wherein said means forautomatically applying comprises means for automatically applyingdifferent post-processing map characteristics inside the region ofinterest than outside the region of interest in response to measuredmotion.
 137. The invention of claim 124, wherein said means forautomatically applying comprises means for automatically applying adifferent ultrasound operating frequency inside the region of interestthan outside the region of interest in response to measured motion. 138.The invention of claim 124, wherein said means for automaticallyapplying comprises means for automatically applying a different transmitpower inside the region of interest than outside the region of interestin response to measured motion.
 139. The invention of claim 124, whereinsaid means for automatically applying comprises means for automaticallyapplying a different logarithmic compression profile inside the regionof interest than outside the region of interest in response to measuredmotion.
 140. The invention of claim 124, wherein said means forautomatically applying comprises means for automatically applying adifferent number of receive line per transmit line inside the region ofinterest than outside the region of interest in response to measuredmotion.
 141. The invention of claim 124, wherein said means forautomatically applying comprises means for automatically applying adifferent transmit pulse shape inside the region of interest thanoutside the region of interest in response to measured motion.
 142. Theinvention of claim 124, wherein said means for automatically applyingcomprises means for automatically receiving a different frequency bandinside the region of interest than outside the region of interest inresponse to measured motion.
 143. The invention of claim 124, whereinsaid means for automatically applying comprises means for automaticallyreceiving a harmonic frequency band inside the region of interest andreceiving a fundamental frequency band outside the region of interest inresponse to measured motion.
 144. The invention of claim 124, whereinsaid means for automatically applying comprises means for automaticallyapplying a different persistence level inside the region of interestthan outside the region of interest in response to measured motion. 145.A method of automatically altering an operating mode of an ultrasoundtransducer array in response to an absence of transducer motioncomprising the steps of:(a) measuring motion of the transducer array bymeasuring motion of a sub-block of pixels between at least twoultrasound-image frames; and (b) automatically altering an operatingmode of the transducer array in response to an absence of measuredmotion.
 146. A method of automatically altering an operating mode of anultrasound transducer array in response to an absence of transducermotion comprising the steps of:(a) measuring motion of the transducerarray by performing one-dimensional correlations between successivelyacquired acoustic lines to determine a position of peakcross-correlation; and (b) automatically altering an operating mode ofthe transducer array in response to an absence of measured motion. 147.The method of claim 145, wherein step (b) comprises the step of removingpower from the transducer array in response to an absence of measuredmotion.
 148. The method of claim 145, wherein step (b) comprises thestep of disabling imaging modes that cause elevated temperatures in thetransducer array in response to an absence of measured motion.
 149. Themethod of claim 145, wherein step (b) comprises the step of operatingthe transducer array in a mode that acquires real image data at a lowerrate in response to an absence of measured motion.
 150. In an ultrasoundimaging system comprising a transducer array, a transmit beamformer, anda receive beamformer, the improvement comprising:means for measuringmotion of the transducer array by measuring motion of a sub-block ofpixels between at least two ultrasound-image frames; and means, coupledto the transducer array and responsive to said means for measuringmotion, for automatically altering an operating mode of the transducerarray in response to an absence of measured motion.
 151. The inventionof claim 150, wherein the means for automatically altering comprisesmeans for removing power from the transducer array in response to anabsence of measured motion.
 152. The invention of claim 150, wherein themeans for automatically altering comprises means for disabling imagingmodes that cause elevated temperatures in the transducer array.
 153. Theinvention of claim 150, wherein the means for automatically alteringcomprises means for operating the transducer array in a mode thatacquires real image data at a lower rate.
 154. A method for generating adistortion-corrected image inside a region of interest in response tomeasured image or transducer motion comprising the steps of(a) measuringmotion; and then (b) automatically generating a distortion-correctedimage inside a region of interest in response to the measured motion.155. The method of claim 154, wherein motion is measured in step (a) bya motion sensor in a transducer.
 156. The method of claim 154, whereinmotion is measured in step (a) by measuring motion of a sub-block ofpixels between at least two ultrasound-image frames.
 157. The method ofclaim 154, wherein step (b) comprises the steps of:(b1) estimating aneffect of the measured motion on line spacing; and (b2) reprocessingline data with corrected line spacing in response to motion measured instep (a).
 158. The method of claim 154, wherein step (b) comprises thestep of repositioning at least one sub-block of pixels in response tomotion measured in step (a).
 159. The method of claim 154, wherein theregion of interest in step (b) comprises a portion of an entireultrasound image.
 160. The method of claim 154, wherein the region ofinterest in step (b) comprises an entire ultrasound image.
 161. Themethod of claim 154, wherein the region of interest in step (b) ismanually selected by a user.
 162. The method of claim 154, wherein theregion of interest in step (b) is automatically selected.
 163. In anultrasound imaging system comprising a transducer array, a transmitbeamformer, and a receive beamformer, the improvement comprising:means,responsive to the receive beamformer, for measuring motion; and means,responsive to said means for measuring motion, for automaticallygenerating a distortion-corrected image inside a region of interest inresponse to measured motion.
 164. The invention of claim 163, whereinsaid means for measuring motion comprises means for determining motionof a sub-block of pixels between at least two ultrasound image frames.165. The invention of claim 163, wherein said means for measuring motioncomprises an image sub-block selector responsive to the receivebeamformer, a motion estimator responsive to the image sub-blockselector, a motion scaler responsive to the motion estimator, and animage interpolator responsive to the motion scaler.
 166. The inventionof claim 165 further comprising a motion smoother responsive to themotion estimator and coupled to the motion scaler.
 167. The invention ofclaim 163, wherein said means for measuring motion comprises a motionsensor responsive to the transducer.
 168. The invention of claim 163,wherein means for automatically generating comprises means forestimating an effect of the measured motion on line spacing and meansfor reprocessing line data with corrected line spacing in response tomeasured motion.
 169. The invention of claim 163, wherein means forautomatically generating comprises means for repositioning at least onesub-block of pixels in response to measured motion.